The use of a light source and some form of spatial light modulator (SLM) to create and then display an image has a variety of applications. Perhaps the most obvious one is a projector, in which the modulation pattern on the SLM represents some form of visual image, and an optical system is then used to project this image onto a screen. Typically the screen is very much larger than the SLM, so the optical system has high magnification in order to achieve this. Therefore the numerical aperture (NA) of the illumination pathway of the SLM is correspondingly higher than that of the screen, since the NA is inversely related to the magnification. In practice, the NA needs to be as high as possible in order to achieve a sufficiently bright screen image.
The two major SLM technologies are digital mirror devices (DMDs), in which each pixel is a miniature mirror that can be rapidly tilted so as either to reflect the illuminating light towards the screen or not, and liquid crystal devices (LCDs), in which each pixel can be controlled so as either to rotate the polarisation of the illuminating light or not, which determines whether or not the light can then reach the screen via a subsequent analyser. Both systems work well when used in conjunction with high NA optics, but DMDs have a problem when the illumination pathway has low NA, as it generally does in microscopy applications, for example. This is because the array of tilted mirrors also acts as a two-dimensional diffraction grating, causing the light leaving the SLM to spread over a larger range of angles than expected from simple reflection. If the subsequent optical pathway has low NA, then much or all of the diffracted light will be lost. Their use in microscopy applications has been described (Krause U.S. Pat. No. 5,587,832, Max Planck EU patents 0911667 and 0916981), but the diffractive losses make them relatively inefficient.
LCDs do not suffer from this problem, but there is now the complication that they require polarised light for their operation, and this invention was devised to address the issues that arise from this requirement. Most light sources are unpolarised, so their output must first be converted to a polarised form. Unpolarised light can be converted into linearly polarised light either by using a filter that transmits only one polarisation, or by using a beamsplitter which transmits one polarisation and reflects the other, thereby allowing both polarisations to be recovered. In both cases this normally reduces the maximum efficiency to 50%, as light of the other polarisation is simply wasted. This is clearly undesirable when the light levels are already limiting.
Various systems have been previously described in which the other polarisation can also be used in LCD applications. These operate by converting it to light of the required polarisation, e.g. by using a half-wave plate, which has the property of rotating linearly polarised light by 90 degrees. The two beams can now be combined, but this is not necessarily an ideal solution. Although one is now using all the light, the overall beam diameter is greater than if the light had all been of the required polarisation to start with. Specifically, if the composite beam is refocussed to produce an image of the light source, there will actually be a separate image for each polarisation, so the radiant intensity (i.e. the light intensity per unit solid angle per unit area of the source) will still be halved from its original value. All such systems will suffer from this problem.
In many applications the loss in radiant intensity may not be important. A true point source, which would have infinite radiant intensity, could be used to generate a perfectly parallel beam, whereas practical light sources, being of finite size, will generate beams that will diverge with distance. In high NA systems such as projectors, the subsequent optics are still likely to be able to accept all the light, in which case there is no problem. However, it is likely to be a problem for applications operating at relatively low NA at the SLM. Low NA here is actually essential in applications, such as microscopy, where a demagnified image of the SLM is to be projected onto an object, since the demagnification will increase the NA at the image to a very high value, thereby limiting the maximum permissible NA at the SLM. This in turn limits the effective size of the light source that can be used for efficient illumination, as only a limited amount of beam divergence can be tolerated.
This invention describes an alternative approach to polarisation recovery, that actually takes advantage of the low NA at the SLM in such applications.